Most cattle have some worms or gastro-intestinal nematodes in their digestive tract. Consider economics and your type of cattle management system when developing a control strategy for your herd.
The most economically important gastro-intestinal nematodes are small, thread-like worms that are located in either the abomasum or intestinal tract of cattle. The life cycles of the major species are similar. Economically important in the Midwest are Ostertagia, Haemonchus, and Trichostrongyles.
Adult worms in the stomach or intestinal tract lay millions of microscopic-sized eggs that are passed in the feces. Under favorable pasture conditions, these eggs hatch and the worms mature through three larval stages. Stage three larvae are ingested, invade the wall of the digestive tract, and become fourth stage larvae, which eventually mature into adult worms. In some worm species, especially Ostertagia, which is the most important nematode of cattle, the larvae may lie in the lining of the digestive tract in an arrested state for long periods of time before becoming mature adults. In northern states this generally occurs during autumn and winter. It is a mechanism that nature uses to ensure the survival of the parasite.
Arrested stage four larvae do not mature until near calving, when they rapidly develop into egg-laying adults and heavily contaminate spring pastures. Heifers may develop clinical disease, including diarrhea, loss of weight and anemia. This phenomenon is referred to as Type II Ostertagiasis. Young grazing calves are very susceptible to worm infection and may become heavily infected. This delayed maturation of stage four larvae ensures heavy pasture contamination in the spring and is a major factor that must be considered when developing worm control strategies.
Under ideal conditions, worm eggs shed on pasture will mature to the stage three larvae in as little as three weeks. Rapid development and maturation of the larvae occurs in warm, moist conditions. Thick, dense pasture growth is conducive to this rapid development. In northern states, many larvae can survive winter as eggs, which hatch in the spring and become infective larvae.
There is a tendency to regard all poor-doers as wormy cattle. Such problems can also be due to infectious diseases, management, nutritional problems, or toxins. Accurate diagnosis is important to avoid costly, unnecessary deworming of cattle. Veterinarians can microscopically analyze fecal samples for parasites using a procedure called fecal flotation. On a herd basis, fecal examinations of suspected wormy cattle are helpful, but do not reliably indicate the degree of parasitism of individual animals because egg production of adult worms is extremely variable. Careful examination of the herd, multiple fecal examinations, ration analysis, blood tests, and post mortem examination of any dead cattle all help determine the level of worm infection in a herd.
The economic effect of low levels of parasitism in cattle is difficult to determine and researchers disagree about the importance of it. Eggs passed by adult cows can contaminate pastures, but the level of pasture contamination caused by calves and yearlings is more significant. Weather and pasture conditions can affect the risk of parasitism. All producers should develop a worm control program that fits their management situation. Management practices and strategic use of anthelmintics (dewormers) are equally important for parasite control.
Beef Cow/Calf Herds
Some confusion exists about pasture rotation and worm control. Rapid pasture rotation (every few weeks) is good pasture and nutritional management, but does not provide parasite control. In northern latitudes, deworming in mid-summer when calves are moved to a new pasture is often cost effective. This removes worms that are in the animals and allows them to graze relatively worm-free pastures.
Continuous use of low-level anthelmintics in creep feed or similar formats may help reduce parasite infection, but should not replace individual treatment when calves are weaned and pre-conditioned.
Calves pastured during the spring and summer are highly susceptible to parasites. Ideally, winter and spring born heifers should be dewormed three and six weeks after turnout to pasture in the spring. This gets rid of larvae that have over-wintered on pasture and are being picked up by the calves. It is a good idea to do a third deworming in mid-summer or early fall when heifers are moved to a different pasture. Yearling heifers should be dewormed in the spring and again several weeks prior to breeding. Move heifers to fresh, lush pastures prior to breeding and deworm at the same time to avoid heavy pasture contamination. Heifers held and bred in dry-lot situations do not need to be dewormed as frequently, but should be dewormed when moved from pasture to dry lot. Deworm heifers about one month prior to calving, regardless of housing and management systems.
Clinical signs of digestive inefficiency are evident in young cattle with acute liver disease and in older cattle with chronic liver disease. Fluke infected cattle show signs similar to those with malnutrition and stomach worms.
Transmission is most common during rainy times in spring and fall. The diarrhea caused by coccidia may be confused with the diarrhea caused by stomach worms, bacteria, and viruses.
Dewormers are ineffective against coccidia. Effective drugs are amprolium, decoquinate, lasalocid and sulfonamides.
Common external parasites include horn flies, lice, and grubs.
Treatment is economically justified when horn fly populations reach 250 per head. To control them satisfactorily throughout the season, use self-treatment insecticides or routinely apply spray, pour-on, spot-on, or dust chemicals.
Used properly, self-treatment devices are more effective than hand application in controlling horn flies and lice. Such devices include oil back rubbers, dust bags and tubes, liquid wicks, and impregnated ear tags. Insecticide-impregnated ear tags control horn flies well for two to five months.
Biting lice and blood-sucking lice are transmitted between cattle by contact, especially in the fall, winter, and spring when egg production increases in cool weather. Because cattle tend to bunch up more in cold weather, uncontrolled lice spread easily from animal to animal and quickly infest an entire herd.
Clinical signs are dry, scaly skin, hair loss, and itching exhibited by biting, rubbing, and scratching. Lice bites and allergies to lice cause the itching. The allergic reaction may persist after the lice are gone. These signs may be confused with malnutrition and allergies caused by horn flies, mosquitoes and gnats.
Although chemicals do not harm lice eggs, cattle can be treated effectively by administering a spray, dust, pour-on, spot-on, or injection. Injection does not work for biting lice.
To control grubs, administer systemic organophosphate insecticides to cattle no later than three months before grubs appear in the back. Use pour-on, spot-on, spray, or injection methods to kill migrating grubs before they reach the esophagus. If cattle are not treated for grubs in the summer, the systemic organophosphate insecticides and ivermectins used in the fall and winter for control of lice, horn flies, and internal parasites may cause reactions in the esophagus if many grubs are present.